Python buttons not appearing in Blender
That's because context.active_object.location=(0,0,0) is not an operator. It's a statement.

You could define your own operator:

import bpy
def main(context):
    # This is what happens when your operator is invoked.
    context.active_object.location = (0, 0, 0)

class SnapToCenterOperator(bpy.types.Operator):
    """Snaps an object to the center."""
    bl_idname = "view3d.snap_to_center_operator"
    bl_label = "Snap Object to Center"

    @classmethod
    def poll(cls, context):
        # This is called to determine whether the operator can be invoked
        return context.active_object is not None

    def execute(self, context):
        # This invokes the operator and returns a state
        main(context)
        return {'FINISHED'}

def register():
    # If it's an addon, this is called when the addon is enabled
    bpy.utils.register_class(SnapToCenterOperator)
def unregister():
    # If it's an addon, this is called when the addon is disabled
    bpy.utils.unregister_class(SnapToCenterOperator)

if __name__ == "__main__":
    # We use this to register it in case we're running it manually
    # (which we are; click the "Run Script" button)
    register()


and then use

row.operator("view3d.snap_to_center", text="Origin")


to achieve the desired result.



NOTE: This is just an example on how to create your own operator. In this case you'll actually just want to use bpy.ops.object.location_clear, as such:

row.operator("object.location_clear", text="Origin")